This invention relates to a novel method of fabricating a phosphor screen of a color television picture tube.
A color cathode ray tube is widely applied in a color television receiving set installed in homes, work sites or public facilities. The color cathode ray tube is designed to produce a color picture by forceful and selective irradiation of electron beams and the color light emission of a patterned phosphor layer deposited in the form of dots, matrix or stripes on the inner wall of a face plate of a color picture tube. Needless to say, the ordinary color television receiving set is provided with a phosphor layer capable of issuing light outputs having the three primary colors of red, greed and blue.
Hitherto, the phosphor screen of the color television picture tube has been fabricated by photographic light exposure. Namely, the customary process of preparing the phosphor screen is to apply proper surface treatment to a glass face plate on which a phosphor screen is to be deposited, develop a prescribed pattern by light exposure on a film of photosensitive solution prepared from polyvinyl alcohol ammonium bichromate, scatter black light-absorbing powder such as graphite on said photosensitive film and provide a light absorbing layer by selective photoetching. For example, the conventional slurry process comprises the complicated steps of preparing a slurry by dispersing phosphor powder in a solution sensitized by polyvinyl alcohol bichromate; applying said slurry on the inner wall of a face plate of a color picture tube, followed by drying, light exposure, development and again drying; and repeating these steps three times to provide phosphors emitting light beams having the three primary colors of red, green and blue. The slurry process comprises many steps and is of the wet type, presenting difficulties in respect of stability, cost and disposal of waste liquid. Demand has already been made to develop a much simpler process of fabricating a phosphor screen and in consequence a color picture tube, which is free from the aforesaid drawbacks. A printing method of impressing the pattern form of ink on the face plate of the color picture tube from an engraved intaglio block has been cited as a prospective means for attaining the abovementioned object. However, the customary printing method raised, as described below, too many problems in fabricating a phosphor screen.
To begin with, a high precision pattern is required for phosphor layers emitting light outputs having the three primary colors of red, green, and blue or a light-absorbing layer. Moreover, an object of impression is not a sheet of paper, but often the face plate of a picture tube, that is, a plate of hard glass. Further, the face plate generally has a curved and moreover concave surface, though sometimes having a flat plane. The glass face plate which can not occlude ink unlike paper and whose plane of impression is not flat presents considerable disadvantages in printing. A phosphor screen poses further problems as described below. The respective phosphor layers which have to emit sufficient luminance must be formed with a considerable thickness, as more than 15 microns, because the particle size distribution of color television phosphors is concentrated at the range of 10 to 15 microns. Even a light-absorbing layer should have a larger thickness than 5 microns. Referring to printing ink, it is desired that to elevate the effect of shutting off the respective phosphor layers or causing them to display full luminance, printing ink be made into such type as contains a far larger amount of phosphor particles (corresponding to pigments in the case of ordinary printing ink) or a light-absorbing material than customary printing ink. However, such type of printing ink lacks fluidity and results in an irregular impression. Where the phosphor layer still contains an unduly large amount of resinous component immediately after printing, then the excess resin is left unremoved even during the subsequent baking step, tending to soil a picture tube when it is put into operation. Further, decomposed gases of the resin evolved during the baking step give rise to cracks or blisters in a metal back layer (a light reflection layer) deposited on the phosphor layer, thus producing a disqualified color television receiving set quite unadapted for practical application. Though, therefore, it is necessary to apply printing ink containing a smaller amount of resin, yet such type of ink presents difficulties in patternization by the ordinary printing method.
Further description is given of some concrete printing methods developed particularly for the above-mentioned purpose. The screen process is considered suitable to print ink with a proper thickness. Since, however, the screen printing process causes ink to be applied on the face plate through a mesh, a printed pattern presents irregular edges. Where a phosphor layer is formed of fine lines as narrow as 0.1 to 0.3 mm, the screen process has the drawback that a printed pattern is produced in broken lines. With the screen process, the mesh is forcefully rubbed by, for example, a squeegee. In this case, the mesh tends to be extended, whether made of synthetic fiber or stainless steel, and is subject to limitation in resistance to printing pressure. Further, possible deformation of the phosphor layer pattern during printing has also to be taken into account. Therefore, the screen process makes it impossible not only to carry out multicolor printing of two or three colors but also to reproduce the details of a phosphor layer pattern with high precision.
The so-called octopus head printing method may be cited for the object of printing a phosphor layer on the curved surface of the face plate. However, the octopus head which transfers ink held in an intermediate position to the face plate is depressed by horizontally applied pressure, thus causing ink to be transferred at an uneven printing pressure from the block via the octopus head to the face plate with the resultant failure to attain accurate and uniform transcription of the patterned form of ink over the whole of an impression to be made. Further, the octopus head is made of relatively soft material with a large thickness. Where, therefore, transcription has to be made over a large area, then the resultant impression is subject to prominent deformation, leading to a noticeable decline in the positional precision of a transcribed pattern.